The spinal column is a complex system of bones and connective tissues that provide support for the human body and protection for the spinal cord and nerves. The adult spine includes an upper portion and a lower portion. The upper portion has twenty-four discrete bones, which are subdivided into three areas including seven cervical vertebrae, twelve thoracic vertebrae, and five lumbar vertebrae. The lower portion has the sacral and coccygeal bones. The cylindrical shaped bones, called vertebral bodies, progressively increase in size from the upper portion downwards to the lower portion.
An intervertebral disc along with two posterior facet joints cushion and dampen the various translational and rotational forces exerted upon the spinal column. The intervertebral disc is a spacer located between two vertebral bodies. The facets provide stability to the posterior portion of adjacent vertebrae. The spinal cord is housed in the canal of the vertebral bodies. It is protected posteriorly by the lamina. The lamina is a curved surface with three main protrusions. Two transverse processes extend laterally from the lamina, while the spinous process extends caudally and posteriorly. The vertebral bodies and lamina are connected by a bone bridge called the pedicle.
The spine is a flexible structure capable of a large range of motion. There are various disorders, diseases, and types of injury, which restrict the range of motion of the spine or interfere with important elements of the nervous system. The problems include, but are not limited to, scoliosis, kyphosis, excessive lordosis, spondylolisthesis, slipped or ruptured disc, degenerative disc disease, vertebral body fracture, and tumors. Persons suffering from any of the above conditions typically experience extreme and/or debilitating pain, and often times diminished nerve function. These conditions and their treatments can be further complicated if the patient is suffering from osteoporosis, or bone tissue thinning and loss of bone density.
There are many known spinal conditions, e.g., scoliosis, that require the imposition and/or maintenance of corrective forces on the spine in order to return the spine to its normal condition. As a result, numerous devices (e.g., alignment systems) have been developed for use in spinal fixation. One type of spinal construct may include, for example, one or more spinal rods that can be placed parallel to the spine with spinal fixation devices (such as hooks, screws, or plates) interconnected between the spinal rods at selected portions of the spine. The spinal rods can be connected to each other via cross-connecting members to provide a more rigid support and alignment system.
When a spinal rod is used as a support and stabilizing member, commonly, a series of two or more screws are inserted into two or more vertebrae to be instrumented. A spinal rod is then placed within or coupled to the heads of the screws, or is placed within a connecting device that links the spinal rod and the heads of the screws, and the connections are tightened. In this way, a rigid supporting structure is fixed to the vertebrae, with the spinal rod providing the support that maintains and/or promotes correction of the vertebral malformation or injury.
Some spinal fixation devices allow one or more degrees of freedom between a fastening portion and a receiving portion of the spinal fixation device, thereby reducing the required precision of placement of the spinal fixation device. The receiving portion of the spinal fixation device may be multi-axially or polyaxially positionable. The receiving portion can be positioned so as to easily receive a spinal rod, limiting or removing much of the positioning difficulty inherent in prior devices. However, such devices provide a maximum angle on the order of 45 degrees between the receiving portion of the spinal fixation device with respect to a longitudinal axis of the fastening portion of the spinal fixation device.
The surgeon attaches the spinal fixation devices to the spine in the appropriate anatomical positions then attaches the spinal rod to the spinal fixation devices. In conjunction, the surgeon manipulates the spinal column and/or individual vertebra to provide the desired treatment for the spinal defect. Subsequently, the spinal rod and fixation devices are locked in a desired arrangement.
While the aforementioned spinal fixation devices are suitable for the above uses, a need exists for a spinal fixation device that can provide a larger degree of angulation between a receiving portion of the spinal fixation device with respect to a longitudinal axis of the fastening portion of the spinal fixation device to facilitate the manipulation of the spine and reduction of various spinal deformities, especially those related to scoliosis.